1. Field of Applicable Technology
The present invention relates to structures and methods of manufacture for an electron emission element which can be utilized as a source of electron emission, for use in various types of apparatus which utilize an electron beam, such as electron microscopes, CRTs, etc. In particular, the invention is directed towards improved structures and methods of manufacture for thin-film electron emission elements which are basically of a metal - insulation - metal layer configuration.
2. Prior Art Technology
In the prior art, a heated cathode is used for electron emission in equipment which uses an electron beam, such as electron microscopes and CRTs. However, with a heated cathode it is of course necessary to provide heating means to produce electron emission, and this has the disadvantage of high energy consumption. For this reason, various types of electron emission elements have been researched which provide electron emission without heating, i.e. cold-cathode electron emission elements.
As a specific example, if a reverse bias voltage is applied to a PN junction, then an avalanche condition can be produced whereby electron emission from the PN junction can be obtained. Alternatively, a localized high-intensity electric field can be applied to a metal electrode, causing field-effect electron emission to occur. Another method is to use a device having a metal - insulator - metal layer configuration (i.e. an MIM type of electron emission element), where the insulating layer and one of the metal layers are respectively formed as extremely thin films, and with the thin metal layer being disposed in a vacuum, and to apply a voltage between the two metal layers whereby electrons execute tunnelling through the insulating layer and a proportion of these are then emitted from the thin metal layer. Of the above types of cold-cathode devices, the MIM electron emission element has the advantage of a simple type of construction, and will be described in the following.
The basic principles of an MIM electron emission element are illustrated in FIG. 1. A metal layer (conducting material layer) 41 has a very thin insulating layer 42 formed thereon, and a very thin metal layer 43 is formed upon the insulating layer 42. The upper surface of the metal layer 43 is exposed to a vacuum, or low pressure gas. By applying a voltage from a power source 44 between the metal layers 41 and 43, having a value that is greater than the work function of the metal layer 43, electron tunnelling through the insulating layer 42 will occur. Some of the tunnelling electrons will have a greater energy than the vacuum potential, and so are emitted from the surface of the metal layer 43, as emitted electrons 45.
FIGS. 2 and 3 show respective specific examples of a prior art type of MIM electron emission element. With the electron emission element of FIG. 2, a metal layer 52 consisting of Al and a metal layer 55 consisting of Au are successively formed on a surface of a glass substrate 51. An insulating layer 52, consisting of Al.sub.2 O.sub.3 and an electrically insulating layer 54 consisting of SiO.sub.2 are formed between the metal layers 52 and 55. When a voltage is applied between the metal layers 52 and 55, electrons are emitted from the electron emission region 56 of the metal layer 55. This is described in the Electronic Apparatus Reasearch Conference Papers of the Television Society, with the title "Cathode Ray Tube Using Tunnel Cathode", 1968, 4, 30.
With the prior art MIM electron emission element of FIG. 3, the metal layer 62 is formed as a strip upon the surface of an electrically insulating layer 61, then an electrically insulating layer 63 is formed on the metal layer 62. Next, a metal layer 64 is formed upon the insulating layer 63, also in the shape of a strip, positioned such as to perpendicularly intersect the metal layer 62. When a voltage is applied between the metal layers 62 and 64, electrons are emitted from an electron emission region of the metal layer 64, with the area of this electron emission region being defined as the region of intersection between the two metal layers. This electron emission element is described in Japanese Patent Laid-open No. 63-6717.
However with the above types of prior art MIM electron emission element, the problem arises that the electron emission distribution within the electron emission region is non-uniform, i.e. there are positions within the electron emission region at which the rate of electron emission is high, and regions where emission is low. Furthermore, with such a prior art type of electron emission element, in addition to the problem of unevenness of electron emission, the metal layer that is formed on the insulating layer will in many cases have poor electrical conductivity.
The reason for the non-uniformity of electron emission distribution is as follows. In order to maximize the efficiency of electron emission from the metal layer that is formed on the insulating layer, that metal layer must be made extremely thin within the electron emission region. However since it is difficult to form the electron emission region with a very uniform thickness of thin metal film, there will inevitably be some variations in thickness within the electron emission region. This results in non-uniformity of electron emission distribution in the electron emission region. Furthermore, due to the fact that the metal layer is very thin, successive voltage drops will occur within that layer, from the point of connection of the metal layer to a power source. Thus, the effective electric field strength within the electron emission region will be non-uniform, causing the electron emission to be uneven within that region. In addition, those electrons which are unable to leave the metal layer by being emitted therefrom, after having passed through the insulating layer by tunnelling, will produce a flow of current within the metal layer. The greater the level of this current flow, i.e. the lower the efficiency of electron emission of the element, the greater will be an amount of Joule heating that is produced in the thin metal layer, whereby heat is generated within portions of that layer. As a result, it becomes impossible to apply a stable and uniform electric field to the insulating layer within the electron emission region, whereby uniformity of electron emission is prevented.
The reasons for the poor conduction of the metal layer are as follows. With the electron emission element of FIG. 3, there are step variations in the height of the surface of the insulating layer, and corresponding step changes are produced in the metal layer that is formed on the insulating layer. Due to these step changes in shape of the layers, corners are formed, and as a result of these corners formed in the layers, defects are produced in the thin metal layer, so that poor electrical conduction in that layer will occur.
Another problem which arises with such a prior art type of MIM electron emission element is that the electron emission efficiency is insufficient. The insulating layer is formed in the prior art as a very thin film, having a thickness of approximately 50 to 200 .ANG., by a process such as evaporative deposition, anodic oxidation, etc. and is not formed with a crystal structure, i.e. is an amorphous layer. As a result, most of the electrons which move within the insulating layer by the tunnelling effect will be dispersed within that layer as a result of collisions with atoms of the material constituting the layer, and will thereby lose energy, so that the number of electrons which actually are transferred to the thin metal layer of such an electron emission element will be very small.
In addition, the electrons which enter this thin metal layer will also be dispersed, and suffer a further energy loss thereby. In order to reduce this dispersion within the thin metal layer, that layer must be made as thin as possible. However if that layer is made very thin, then a substantial voltage drop will occur between the point of connection of that metal layer to a power source terminal and an electron emission region of the metal layer, as described above. Thus, the intensity of the electric field that is produced in a region of the insulating layer immediately below an electron emission region of the thin metal layer is reduced, and hence effective electron emission cannot be achieved. This reduction of effective electric field strength within the electron emission region can be counteracted to some extent by increasing the level of voltage supplied by the power source. However in that case, Joule heating will occur due to the electrical power which will be dissipated within the metal layer, resulting in non-uniform electron emission and the danger of open-circuits in the thin metal layer.
Furthermore, with such an MIM type of electron emission element, both the thin-film insulating layer (through which the tunnelling electrons pass) and the thin-film metal layer (from which electrons are emitted) are formed upon an underlying layer which has an upwardly protruding portion and/or an inwardly recessed portion, i.e. which has an upper surface that exhibits step changes in height. As a result, each of the thin insulating layer and thin metal layer will also have corresponding step changes in height, so that the respective film thicknesses of these layers will be non-uniform. Hence, the operating characteristics of such an electron emission element are unstable. In particular, if the thin metal layer is formed with such step variations in height, then internal defects may be produced within that layer in regions which are close to these step changes, i.e. at corner portions. This can result in localized variations in electrical conduction at these portions, as well as non-uniformity of film thickness, thereby further contributing to deterioration of the electrical characteristics. Moreover, if an array is formed of a plurality of such MIM electron emission elements, the characteristics of respective elements of the array will vary significantly from one another.